What parasites a person would expect to host, ignoring bacteria, would vary tremendously based on soil types, rainfall, host genetics, food gathering or farming practices, climate, etc.
But in a lifetime it was typical to be constantly infected with a variety of helminths and protozoa. There was an interesting, if speculative, paper positing that the rise in colorectal cancer was linked to clean drinking water, specifically the elimination of the protozoa Eimeria. It was just epidemiological evidence but it was compelling.
Basically the numbers and variety of parasitic, commensal and mutualist symbionts we are exposed to, the most important probably being in childhood, have been substantially reduced. We are also no longer required to "tough it out" with infections, antibiotics have put paid to that. So that even non-lethal infections are not allowed to run their course.
Then there are all the vaccines, the number given before age six routinely in the US is ridiculous already (Chicken pox, really?) and set to DOUBLE in the next decade or so. And these are mandatory. When my kids were in school you could refuse for religious reasons (in my opinion superstition) but not on the basis of reason or science... That is your kid would be excluded from school if you did not get them all the shots, including chicken pox.
Then there are all the lame ads for hand sanitisers, etc.
The irony is that while everyone worries about the "environment" as something over there and separate from them, natural selection through the destruction of our personal ecosystems is already sickening and shortening the lives of literally hundreds of millions of people world wide. People who represent the very latest genetic model, in Palaeolithic terms.
Crohn's, UC and Coeliac disease have all been shown to arise out of polymorphisms that have been shaped by coevolution with parasites, and in particular helminths for instance. People are getting sick because they are so well adapted to parasites, but the immuno modulatory effect is absent, because they are.
Obviously I could go on all day.